1. Field of the Invention
The present invention relates to a signal reception device, a signal transmission device, a radio communication system, and a signal reception method used in mobile wireless communications and fixed wireless communications.
2. Description of the Related Art
In a wireless communication system, in order to achieve efficient utilization of limited frequency resources, it is desirable that signals use the same frequency as much as possible. As achievement of research and development in response to this requirement, for example, an interference canceller is attracting attention as a technique able to increase capacity of a communication system.
FIG. 25 is a block diagram illustrating an example of a configuration of an interference canceller of the related art.
The interference canceller 200 as illustrated in FIG. 25, which is installed in a signal receiver of a wireless communication system, generates a received signal replica by combining the replicas of signals in a received signal, separates a desired signal from other signals (namely, interference signals), and extracts the desired signal. Such an interference canceller 200 is called “replica generation type interference canceller”.
Below, operations of the replica generation type interference canceller 200 are described with reference to FIG. 25. For convenience of description, it is assumed that there are two received signals (signal SIG1 and signal SIG2).
In the replica generation type interference canceller 200, a channel estimator 218 estimates channel impulse responses of the signals in consideration of the time spread of transmission paths of the desired and the interference signals. Coefficient variable filters 214 and 215 generate the replica signals of the desired and interference signals for all possible symbol sequence candidates of the desired and the interference signals by taking convolution of the estimated channel impulse responses (estimated values of the channels) and the symbol sequence candidates. An adder 216 sums the signal replicas of the desired and the interference signals, and generates a received signal replica. A maximum likelihood sequence estimator 220 selects a symbol sequence candidate of the desired and the interference signals, whose received signal replica is the closest to the actually received signal, and outputs a symbol sequence candidate of the desired signal as the estimation result of the received signal, thus effectively canceling the interference signals.
By adaptively canceling the interference signals in the received signal in this way, it is possible for many signals to use the same frequency at the same time, and to improve frequency utilization efficiency.
Additionally, in Japanese Laid Open Patent Application No. 2000-252958, it is proposed to estimate characteristics of channels in each path corresponding to a user so as to compensate for phases of signals, and to cancel interference by using the compensated signals.
With this technique, by using a frequency offset estimated at a final stage, it is possible to precisely estimate characteristics of channels, and as a result, it is possible to improve quality of demodulated data.
However, although the above interference cancellers of the related art are capable of effectively suppressing interference signals by generating the desired and the interference signal replicas, when center frequencies of the desired and the interference signals are different, it is observed that signals at the receiver after detection rotate at high speeds.
FIG. 26 is a diagram illustrating a problem of the related art when signals having different center frequencies are superposed on the same frequency for transmission.
In FIG. 26, it is assumed that a transmission device T1 and a reception device R1 communicate with each other, and the transmission device T1 transmits a wide band signal SIG1 having a center frequency fc1 and a bandwidth BW1. In addition, it is assumed that a transmission device T2 and a reception device R2 communicate with each other, and the transmission device T2 transmits a narrow band signal SIG2 having a center frequency fc2 and a bandwidth BW2. Under these conditions, focusing on signals received by the reception device R1, as shown in FIG. 26, the reception device R1 receives not only the wide band signal SIG1 from the transmission device T1, but also the narrow band signal SIG2 from the transmission device T2. In other words, as illustrated in the upper part of a portion (a) of FIG. 26, signals having different center frequencies exist on the same frequency band. If these received signals are expanded in an IQ plane, as illustrated in the lower part of the portion (a) of FIG. 26, it is observed that the received signal rotates at high speeds corresponding to the difference of the center frequencies. As a result, when the interference cancellers of the related art are used in such an environment, the channel estimation algorithm cannot follow the high speed rotation, resulting in great degradation of performance in the interference cancellation.
Further, for wireless signals partially sharing or completely sharing the same frequency, if the bandwidths of the wireless signals are different from each other, the difference of the center frequencies of the desired and the interference signals is the major cause of the great degradation of performance in interference cancellation.